Neuronal polarity is critical to proper brain development, but the mechanisms underlying the polarized outgrowth of dendrites and axons remain poorly understood at the molecular level. Understanding the mechanisms patterning neuronal connections is essential to understanding their roles in nervous system function and dysfunction. This proposal aims to identify a critical molecular mechanism specifying the polarity of dendrite/axon growth of pyramidal neurons in the neocortex. We have recently undertaken studies to identify genes specifying neuronal polarity during cortical development. One candidate molecule is LKB1 (also called Par4), a serine-threonine kinase that is a firmly established regulator of cell polarity in invertebrates and mammalian epithelial cells. Loss of LKB1 leads to the cancer predisposition syndrome known as Peutz-Jeghers. Using conditional deletion of LKB1 expression in the developing mouse cortex, we found that LKB1 is required for axon initiation and that over-activation of LKB1 can induce the emergence of multiple axons from the cell soma in developing pyramidal neurons. We propose to use a structure/function approach in order to identify the domains of LKB1 required for initiating axon outgrowth (Aim 1). LKB1 has several identified substrates, many of which are implicated in the establishment of cell polarity. We will then use both gain- and loss-of-function approaches in vitro and in vivo in order to determine which downstream effectors are critical in order for LKB1 to specify neuronal polarity (Aim 2). Finally, combination of time-lapse imaging and live fluorescent reporters of cell polarity will be used to identify when and where LKB1 is required in order to specify axonal polarity of developing cortical neurons (Aim 3).